Method of heat treatment of steel



Patented Oct. 29, 1929 UNITED STATES PATENT OFFICE SOJI MAKITA, OF GHATSU-MACHI, MUROBAN-SHI, HOKKAIDO, JAPAN, ASSIGNOB TO KABUSHIKI KAISHA NIKON SEIKOSHO, OF TOKYO, JAPAN METHOD OF HEAT TREATMENT OF. STEEL No Drawing. Application filed April 19, 1927, Serial No. 185,074, and injapan June 22, 1926.

The invention is a special heat treatment of steel of all kinds, including carbon and alloy steels. i The method is a special heating treatment in three stages. In the first stage, steel is heated above the transition point A and is quenched. In the second stage, the steel is heated up to a temperature a little higher than the point A0,, or thereabout, and cooled down to a temperature a little lower than the point An, or thereabout. In the third stage, following the second, it isheated to a temperature between A0, and A13, and then cooled down to the atmospheric temperature. The new method, covered by the present invention, imparts to the steel under the treatment qualities of far greater superiority for all practical purposes of its technical application, compared with the ordinary annealmg method known to the public.

It gives to steel greater qualities of dead softness, toughness, elasticity and uniformity.

The purposes of this treatment of steel may 'be cited as follows :-(1) to remove internal strains, cause by thermal or mechanical influence, (2) to soften it so as to make it more easily machinable, (3) to secure such desirable strength and at the same time such 1 ductility as may safely withstand any necessary practical strain, when subjected, and especlally to increase elastic limit and impact value, and (4.) to unify and to impart to it such line structure as to make it immune from that weakness, which accompanies any steel of coarse structur 1 For the consummation of these purposes, the only one method that has hitherto been well-known to, and generally adopted by, the public at large, is a heat treatment, wherein steel is slowly cooled down from a temperature above the transition point. i

In this well-knownmethod, however, the crystal structure is apt to aggregate and all the undesirable effects develop during the slow process and are imparted to the steel affecting and impairing its strength to a considrable degree. .Nevertheless, this StI'HC- tural change of steel in quality can not be avoided, inasmuch as it is'the-natural result of this method.

The present invention is an improvement upon the old annealing method, remedying all its defects and increasing the intrinsic valueof the steel under treatment, by removing the internal crystal aggregation and unifying'its structure. The special method of the present invention is to subject steel firstly to quenching and next to special tempering operations twice. Speaking more fully in detail, the methodis, in its first stage, the quenching of steel from a temperature above 'the transition point A and in the second stage, of heating it again'up to a temperature a little higher than the point A0, or thereabout, cooling it down to a temperature a little lower than the point Ar or there about, and in the third stage, of heating it up to a temperature between the" point A0 and Ar 'and keeping'it in the same temperature for some time, and finahy cooling-it down to the atmospheric temperature.

For example, the best result can be obtained by treating steel after quenching, in such a way as to heat it up to a temperature a little higher than the point A0 cooling it down to a temperature a little lower .than the point A7 heating itagain up to a temperature fifidllitlfi lower than the point A0 keeping it in that. same temperature for certain hours and lastly leaving it to cool down.

Here at this point of heating operation, each .or the whole of the three stages above referred to may be suitably repeated, if required. It inust be noted that these modified operations, being nothing more than practical applications of theinvention, should be considered as invention.

Although the new method may appear first to be something similar to the ordinary quenching and tempering treatment, yet these two processes will be found en 'irely dif;

perfectly within the scope of this ferentfrom each other. I Becausethe tempering-operation afterquenching in the latter is 1 only once to reheat steel to :a temperature not exceeding the .point- A and in consequence the extent of the softening eifect can notbe brought beyond that of the ordinary annealing, in which steel is slowly cooled down from a temperature above the transition point. 100

down.

According to the present method, the effect covering the elastic limit, softness, ductility and especially impact values will be found greatly increased, com ared with the effect covering similar-quali ies of steel according to the ordinary annealing. Moreover,

the crystal structure becomes homogencoils and fine granular pearlite, while the aggregated structure of lamellar pearlite is secured by the ordinary annealing method. Therefore, by the present method, not only the purposes of annealing are successfully accomplished, but various superior qualities arealso imparted to the steel.

A like structure of granular pearlite, or divorced pearlite ossessing the qualities of softness and duct' was, it is true, already found, and investigated into by several scientists. But such structure obtained by some other methods, had unfortunately a poor impact value due perhaps to some imperfectness of the structure, making the steel thus treated technically impossible for practical purposes.

The invention is applicable to steel of all kinds, viz, half finished and completely finished products, irrespective of whether cast or forged and of whether carbon or alloy steel.

By the new softening treatment, steel de- 'velo s new properties as described-above,

whic have not been known before. Therefore, the present invention is a new method,

which is entirely different from the ordinary one.

I claim:

A method of heat treatment of steel, consisting in quenching thesteel from a temperature above the transtition point A then heating it up to a temperature a little higher than the point A0,, cooling it down to a temperature a little lower than the point A1,, heating it up to a temperature a little lower than the point Ac,, maintainin that temperature for a period of time, an finally cooling it In testimony whereof I aflix my signature.

SOJ I MAKITA. 

